E-cadherin downregulation occurs frequently in cancer and is clearly a pivotal event in the transition to metastasis. Interestingly, recent studies of the major human tumor types also reveal frequent downregulation ofpl20, but the mechanism and consequences of pl20 downregulaton are unknown. Paradoxically, cancer cell lines that might represent the p 120-deficient condition have not been identified. Moreover, p 120 downregulation in tumors has been largely ignored because until recently there was no compelling reason to focus on the issue. This proposal is based primarily on three key observations, two of which constitute our preliminary data. First, p120 is required for E-cadherin stability. Second, p120 expression is frequently downregulated or regionally absent in a significant subset of the most common human cancers (e.g., colon, prostate, lung, breast, and others). Third, DN-cadherins strongly promote tumor progression and/or metastasis in animal models, and probably act via sequestering p 120. The data provides compelling evidence that p120 downregulation could be the main event leading to E-cadherin downregulation in a large number of tumors. If validated, this concept will radically change how we think about an event that is causally linked to the transition to metastasis in most carcinoma types. Until now, we have not been able to study the p120 phenomenon because we could not adequately model the condition. The mouse renal xenograft model provides a novel method for growing and maintaining human tumors that accurately phenocopy the tumors from which they were originally derived. Importantly, we now have two xenografts (prostate and lung) that are almost completely p120 and E-cadherin negative, and probable access to others. In aim 1, the major objective is to use this system, and possibly 3D-matrigel cultures, to determine whether restoring p 120 expression in these tumors is sufficient to rescue endogenous E-cadherin (and epithelial morphology). A positive result would be extremely significant because these xenografts represent a huge number of similarly disposed human tumors that are not otherwise accessible for testing. In aim 2, we propose to take further advantage of these systems to identify the mechanism of pl20 downregulation in tumors. These concepts may lead to novel approaches aimed at clinical intervention at the level of tumor progression to metastasis, one of the most difficult issues in cancer biology. The matrigel and xenograft models could be outstanding models for future preclinical studies aimed at learning how to turn p 120 (and Ecadherin) back on.